First - velvety dark skin in the groin, neck, armpit area etc could be a sign of diabetes. So make sure you are getting your blood sugar checked at your annual MD visits (as an MD normally would do - nothing special to ask for). Probably not what you have but always better to be overly thorough - so just make sure you are at least getting an annual physical.
SEcond - Like Amandie, I have dark inner thighs also from chub rub and I am guessing this is exactly what you are talking about! I wish I had a solution. Are there skin lightening creams that could help? If it makes you feel better, I had a friend who lost a significant amount of weight and the dark skin from chub rub eventually faded away!!! That's what I've been hoping for!